The present invention relates to a method and device for detecting and controlling the printing pressure, in particular, in flexographic machines.
As is known, flexographic printing machines are usually employed for performing printing, in one or more colours, on films of plastic material, paper or cardboard. For the application of each colour the flexographic machines are usually provided with a first rotating inking roll, or fountain roll, which is partially immersed inside a container with ink inside it, and a second rotating screened roll, or anilox cylinder, which is substantially tangential to the fountain roll and is able to remove at each rotation, during use, a given quantity of ink from the fountain roll itself, a third rotating plate-carrying roll, or plate cylinder, which is provided on its periphery with at least one printing plate, this plate consisting of lowered surface portions and raised surface portions reproducing the motif to be printed, being arranged substantially tangential to the anilox cylinder and capable, at each rotation, of retaining on the surface of the raised portions themselves a given quantity of ink supplied by the anilox cylinder, and a fourth printing roll, or impression cylinder, arranged substantially tangential to the plate cylinder and defining, together with the plate cylinder itself, a narrow passage intended for the transit of a strip of material on which printing is to be performed.
According to another embodiment of the known type, the fountain roll, the ink container and the anilox cylinder may be replaced by a group consisting of an ink container closed by doctor blades and an anilox cylinder which removes the quantity of necessary ink directly from the container.
The mutual distance existing between the plate cylinder and the impression cylinder is considered to be a particularly critical parameter since the result of the printing operation depends on this distance: if this distance differs appreciably from an optimum value, the printing operation performed is of mediocre quality. In practice, this distance may be modified, during operation of the flexographic machine, by temperature variations of the said cylinders, by wear of the cylinders themselves and by deformations in the structure which supports the cylinders themselves, and the optimum value of this distance varies with variation of the said strip of material and the subject to be printed.
In order to verify the printing quality it is known to use telecamera equipment. This equipment, however, in addition to being very costly, in the event of defective printing is unable to eliminate the cause of the malfunction, but merely indicates the malfunction itself to an operator.
In any case, the operator must modify manually the distance between the said cylinders by means of long and extremely critical operations which require a considerable amount of experience on the part of the operator him/herself, also because the procedures for carrying them out depend on the material on which the printing operations are performed and the design to be printed.
These manual adjustments are also required when at the start of a printing operation, for example with a new plate, the distances between the plate cylinder and the impression cylinder must be set.
A further cause of uncertainty able to prevent optimum operation-of the flexographic machines in question arises from the fact that the periphery of the said plate cylinder is defined by at least one printing plate which is fixed to the plate cylinder itself by means of thin bi-adhesive film. This film is subject to variations in thickness following temperature changes and also following the printing action of the cylinder, thereby influencing the printing quality.
A further critical parameter of the said printing machines is the distance which the anilox cylinder and the plate cylinder have between them, since correct inking of the plate also depends on this distance. It is obvious that these distances between the plate cylinder and impression cylinder and between the plate cylinder and anilox cylinder are influenced by too many variables and parameters which in practice are difficult to control.
Consequently, an idea which is proposed is that of changing the parameter on the basis of which detection and control are performed.
In fact, since it is difficult, if not impossible, to constantly control the optimum distance between the cylinders, the parameter "distance between the cylinders" has been replaced by the pressure existing between the plate cylinder and impression cylinder, namely, instead of controlling the position of the cylinders it is proposed to control directly the printing pressure.
Moreover, it is proposed to detect also the pressure existing between the plate cylinder and the anilox cylinder, namely the inking pressure.
In fact, it has been discovered that detection of the aforementioned pressures and controlling thereof within suitable values enable optimum-quality prints to be obtained.
In this way all the variables which modified in an unpredictable manner the distance between the plate cylinders and impression cylinder and anilox cylinder are overcome, since the printing pressure and the inking pressure are controlled directly, these being the parameters which influence most of all the printing quality.
The main problem to be solved with regard to the printing methods in flexographic machines of the known type consists in avoiding variations in the printing quality due to variations in the temperature conditions, the bi-adhesive film for fixing the plate, wear of the plate and wear of the flexographic machine itself and any other parameter which modifies the optimum printing conditions.